Re: [RFC PATCH 1/4] powerpc 2.6.16-rt17: to build on powerpc w/ RT

From: Benjamin Herrenschmidt
Date: Wed Aug 16 2006 - 20:24:31 EST

> Hey Ben,
> I appreciate your looking over my patch. You are correct, the
> conversion is a bit rough and I've not yet been able to work on the
> powerpc vDSO, although I'd like to get it working so any help or
> suggestions would be appreciated (is there a reason the vDSO is written
> in ASM?).
> If you have any other concerns w/ that patch, or the generic timekeeping
> code, please let me know and I'll do what I can to address them.

Well, I've been wanting to look at your stuff and possibly do the
conversion for some time, provided we don't lose performances ... Our
current implementation is very optimized to avoid even memory barriers
in most cases and I doubt we'll be able to be as fine tuned using your
generic code, thus it's a tradeoff decision that we have to do. But
then, I need to look into the details before doing any final
statement :)

As for why the vDSO is in assembly, well... because it's kewl ? :) More
seriously, because it's much more simpler that way (and it's hand
optimized in a couple of places, though that would probably benefit
going through a proper scheduling analysis). The vDSO code has "special"
calling conventions (like the need to tweak, the non-use of the
TOC, the lack of procedure descriptors, symbols are offsets to the
functions, etc...) that makes it awkward to write it in C.


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